Sound translating apparatus



Nov. 10, 1942.

H. F. OLSON SOUND TRANSLATING APPARATUS Filed Jan. 2, 1940 Bnventorattornel Esiented Nov. 1Q, 1942 2,301,638 7 SOUND TRANSLATING APPARATUSHarry I. Olson, Haddon Heights, N. 1., amignor to Radio Corporation ofAmerica, a corporation 01 Delaware Application January 2, 1940, SerialNo. 312,053

GCla-ims.

This invention relates to sound translating apparatus, and moreparticularly to a microphone of the type employing a movable conductorin a magneticileld,thepresentinventionbeingan improvement over thatdiscwed and claimed in Reissue Patent No. 19,115, granted to H. F. Olsonand J. Weinberger.

In the aforesaid patent, there is disclosed a ribbon microphoneemploying two serially connected ribbon oimduetors mounted for movementin a magnetic field, one of said conductors being responsive to thepressure component of a sound wave and having a non-directionalcharacteristic, and the other of said conductors being responsive to thepressure gradient component of the sound wave and having a bidirectionalcharacteristic. The combination of these two conductors provides amicrophone which has a unidirectional characteristic, as represented bya cardioid of revolution, when the sensitivity of each of the conductorsor units is equal.

Now, I have discovered that it is not necessary to employ two separateconductors as aforesaid in order to provide a microphone having aunidirectional characteristic, but that a microphone having such acharacteristic can be made with but a single conductor, and the primaryobject of my present invention is to provide a microphone of this sort.

More particularly, it is an object of my present invention to provide animproved microphone of the type set forth which is extremely simple inconstruction and yet highly eflicient in use.

Another object of my present invention is to provide an improvedmicrophone of the type set forth, the directional pattern of which issuperior to that of previously known unidirectional microphones.

A further object of my present invention is to provide an improvedmicrophone as aforesaid which can readily be changed from a microphonehaving a unidirectional characteristic to one having either abidirectional characteristic or a nondirectlonal characteristic at will.

In accordance with my present invention, I employ but a single conductoror ribbon and place behind it a pipe terminating in a labyrinth filledwith tufts of felt or the like and constituting an acoustic resistance,just as in the case of the pressure responsive section of theconventional microphone shown, for example, in the above noted reissuepatent. However, I provide in the pipe immediately behind the ribbon oneor more openings which can be closed off more or less. when the openingsare entirely closed, the microphone acts as a non-directional pickupdevice. when the openings are entirely open, the microphone acts as abi-directional microphone, since the sound wave energy can reach bothsurfaces of the conductor freely. By closing off the openings onlypartially, the resulting microphone will have either a unidirectionalpattern or one which closely approximates the unidirectional pattern,depending upon the extent to which the openings are closed. A microphoneof this sort affords the advantages of simplifying the magnet structurewhich provides the magnetic field for the ribbon or conductor with aresultant increase in efliciency, simplifies the ribbon mountingproblems, permits reduction in the length of the ribbon, reduces theresponse to wind, and results in a more uniform directionalcharacteristic with respect to frequency.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description of several embodimentsthereof, when read in connection with the accompanying drawing, in whichFigure 1 is a fragmentary sectional view of a ribbon microphone which isresponsive solely to the pressure component of the sound wave and has anon-directional characteristic.

Figure 2 is a similar view of a microphone according to one feature ofmy present invention and wherein the directional characteristic is acardioid of revolution,

Figure 3 is a similar view of a microphone according to another featureof my present invention and wherein the ribbon is responsive to thepressure gradient component of the sound wave with the result that ithas a bidirectional characteristic,

Figure 4 is a similar view of a microphone incorporating the features ofthe microphones of Figs. 2 and 3,

Figure 5 is a central sectional view of a microphone according to myinvention and illustrating the principle of operation thereof,

Figure 6 is a side elevation of a complete microphone formed accordingto my present invention,

Figure 7 is a rear elevation thereof, and

Figures 8 to 11, inclusive, are curves showing the directionalcharacteristics of the various pickup devices to which my improvedmicrophone maybe adjusted.

Referring more particularly to Figs. 1 to of the drawing, I have shown aPair of pole pieces I of a suitable magnetic structure spaced apart toprovide an air gap within which is vibratably mounted a conductiveribbon or'the like 3 of well known form. Behind the ribbon I, there ismounted a closed pipe 5 (Fig. 1) which ter-= minates in a suitablelabyrinth structure or the like i filled with tufts of felt or similardamping material 9, the pipe and labyrinth structure acting as anacoustic resistance terminating the ribbon 3. The ribbon 3 substantiallycloses the mouth of the pipe 5 as shown in the drawing and as more fullydisclosed in the above identified Reissue Patent No. 19,115, forexample. If the pipe 5 is closed at the rear of the ribbon 3, as shownin Fig. 1, sound waveswill strike the ribbon 3 only at the front surfacethereof, as represented by the arrow A (Fig. 5), and the ribbon 3 willbe responsive only to the pressure component of the sound wave. Themicrophone will then have a non-directional characteristic, as shown bythe curve in Fi 11.

If an opening H is formed in the pipe 5 behind the ribbon 3, theresponse characteristic of the microphone will be altered, dependingupon the size of the opening. If the opening ill is small, as shown inFig. 2, then sound coming from the front of the microphone will reachthe ribbon 3 along the paths represented by the arrows A, B in Fig. 5.Obviously, the sound traveling along the path B will reach the back ofthe ribbon 3 considerably later than that traveling along the path Abecause the sound wave traveling along the path B must not only passaround the pipe 5, but must encounter the inertance provided by thesmall opening II as it passes therethrough. Sound coming from-the backof the microphone will take the path C around to the front of the ribbonand the path D to the back of the ribbon. If the delay afforded by thesmall opening II to the sound wave traveling along the path D issubstantially equal to the delay encountered by the sound travelingalong the path C as it passes around the pole pieces I to the front ofthe ribbon (which can easily be the case by properly choosing the sizeof the opening ll), then sound waves traveling along the paths C and Dwill reach the front and rear I surfaces of the ribbon 3 simultaneouslyand in phase. For sound waves traveling at right angles to the axis ofthe microphone, as represented by the arrows E and F, those travelingalong the path E around to the front of the ribbon 3 will reach theribbon slightly in advance of those traveling along the path F to therear of the ribbon since the latter must pass through the opening II.The result is that the microphone has the cardioid characteristic of aunidirectional microphone, as shown by the curve in Fig. 10.

Now, if the opening II is made quite large, there will be practically noacoustical. inertance oilered by the opening ll. However, the soundtraveling along the path B will reach the back of the ribbon somewhatlater than that traveling directly to the front of the ribbon along thepath A because the former must pass around the pole pieces I and a smallportion of the pipe 5. Similarly, sound traveling along the path C willreach the front of the ribbon slightly later than that traveling alongthe path D, while sound traveling along the paths E and F will reach theribbon simultaneously and will cancel out. The result is that themicrophone is responsive to the pressure gradient or velocity componentof the sound wave and it has the bidirectional characteristic shown inFig. 8.

If the opening II is made intermediate in size (i. e., larger than inthe case represented by Fig. 2, but smaller than in the case representedby Fig. 3), the directional characteristic will be altered graduallyfrom that shown in Fig. it: to that shown in Fig. 8. At some point, thesize of the opening ll may be so chosen that the characteristic will bea limacon, for example, as represented by Fig. 9.

In :ig. 4, I have shown a structure wherein the size of the openingbehind the ribbon 3 may be adjusted to give the desired characteristic.Instead of forming but a single opening in the pipe 5, I form aplurality of small openings l3 therein and pivotally mount on the backof the pipe 5 a shutter I5 which is provided with an equal number ofopenings ll. The shutter i5 is so arranged that it may be moved to anyone or more of a number of predetermined positions to cover one or moreof the openings 13, or to the position illustrated in Fig. 4, whereinthe openings i3 are all uncovered by aligning the openings ll therewith.This is. in effect, the same as providing a large opening in the pipe 5,such as that illustrated in Fig. 3.

In Figs. 6 and 7, I have shown a complete microphone embodying theprinciples of my present invention. The magnetic field is supplied by amagnet 18 in engagement with the pole pieces I, and the entire structureis housed within a suitable, perforated casing or cover l9 having anarcuate slot 20 formed in the back thereof. The shutter I5 is pivotallymounted on a pin 2! on the back of the pipe 5 and is formed with anoperating handle 22 extending out through the slot 2.. An index plate 23is mounted on the back of the casing l9 as a reference index for thevarious positions of the shutter l1. -Thus, when the handle 22 is movedto position 1 (the extreme left as viewed in Fig. 7), all of theopenings IS in the pipe 5 are exposed and the microphone becomes avelocity microphone having the bi-directional characteristic shown inFig. 8. When the handle 22 is moved to position 2, one of the openingsl3 becomes covered (as shown by the dotted opening IS in Fig. 7) toprovide a somewhat smaller overall opening in the pipe 5, and themicrophone has the limacon characteristic of Fig. 9. By moving thehandle 22 to position 3, three of the openings l3 in the pipe 5 willbecome closed and only one open, in which case the microphone becomesuni-directional and has the cardioid characteristic of Fig. 10. Finally,by moving the handle 22 to position 4, all of the openings in the pipe 5become closed and the microphone acts as a non-directional pressureresponsive microphone having the characteristic shown in Fig. 11.

From the foregoing description, it will be apparent to those skilled inthe art that I have provided a novel and efficient microphone employingbut a single ribbon, with its inherent advantages, and adjustable to anypreselected one of a plurality of predetermined directionalcharacteristics at will, since the inertance provided by the opening llmay be adjusted until the desired characteristic is obtained. Although Ihave shown and described but one embodiment of my invention, it will beapparent to those skilled in the art that many other modificationsthereof are possible. I therefore desire that my invention shall not belimited except insofar as is made necessary by the prior art and by thespirit of the appended claims.

I claim as my invention:

1. In a sound translating device, the combination of means for producinga magnetic field, an elongated conductor mounted in said field forvibration therein in response to sound wave energy, a pipe mountedbehind said conductor in close proximity thereto and including meansconstituting a terminating acoustic resistance for said conductor, saidconductor substantially closing the mouth of said pipe and said pipehaving an opening therein behind said conductor, and means for more orless closing ofl said opening.

2. In a sound translating device, the combination of means for producinga magnetic field, an elongated conductor mounted in said field forvibration therein in response to'sound wave enorgy, and a pipe mountedbehind said conductor in close proximity thereto and including meansconstituting a terminating acoustic resistance for said conductor, saidconductor substantially closing the mouth of said pipe and said pipehaving a large opening therein behind said conductor whereby sound wavesmay reach said conductor both from the back and from the frontsubstantially freely.

3. In a sound translating device, the combination of means for producinga magnetic field, an elongated conductor mounted in said field forvibration therein in response to sound wave energy, and a pipe mountedbehind said conductor in close proximity thereto and including meansconstituting a terminating acoustic resistance for said conductor, saidconductor substantially closing the mouth of said pipe and said pipehaving an opening therein behind said conductor of a size suflicient toimpart to said device a directional characteristic which is a cardioidof revolution,

4. In a sound translating device, the combina tion of means forproducing a magnetic field, an elongated conductor mounted in said fieldfor vibration therein in response to sound wave energy, and a pipemounted behind said conductor -in close proximity thereto and includingmeans constituting a terminating acoustic resistance for said conductor,said conductor substantially closing the mouth of said pipe and saidpipe having an opening therein behind said conductor of a sizesufficient to impart to said device a directional characteristic whichis a limacon.

5. In a sound translating device, the combination of means for producinga magnetic field, an elongated conductor mounted in said field forvibration therein in response to sound wave energy, and a pipe mountedbehind said conductor in close proximity thereto and including meansconstituting a terminating acoustic resistance for said conductor, saidconductor substantially closing the mouth of said pipe and said pipehaving an opening therein behind said conductor of a 1 size sufficientto impart to said device a cosine characteristic whereby said device isbi-directional in response.

6. In a microphone, the combination of means for producing a magneticfield, a single elongated conductor mounted in said field for vibrationtherein in response to sound wave energy, and a pipe mounted behind saidconductor providing an acoustic resistance to said conductor, saidconductor substantially closing the month of said pipe and said pipehaving an opening therein behind said conductor constituting an acousticinertance.

HARRY F. OLSON.

